High molecular weight, linear polyester resins of the poly(alkylene terephthalate) family are known to be superior components in thermoplastic fibers, films and molding compositions, because of their excellent physical properties such as strength, toughness, and solvent resistance, in combination with their desirable appearance. Among the most useful such polyesters are poly(ethylene terephthalate), poly(1,3-propylene terephthalate), and poly(1,4-butylene terephthalate). Of these, poly(1,4-butylene terephthalate) is particularly valuable because it crystallizes very rapidly from the melt, and thus can be formulated into compositions that are moldable in conventional equipment with conventional temperature and cycle times, without any need for the addition of nucleating agents.
Poly(alkylene terephthalate) resins are commonly prepared by one of two methods. The first method occurs in two stages, and uses a dialkyl terephthalate (DTP) as a starting material. The first stage is transesterification of a lower C1-C6 alkyl terephthalate such as dimethyl terephthalate, with an excess of a diol such as ethylene glycol, 1,3-propanediol, or 1,4-butanediol, together with removal of the alcohol (e.g., methanol) byproduct. In the second stage, high molecular weight polyesters are produced by polycondensation of the intermediate product under vacuum and at high temperature, together with removal of the excess diol.
The second method uses terephthalic acid (TPA) as a starting material, and is also a two-stage process. The first stage is direct esterification of terephthalic acid using an excess of the diol in a heterogeneous reaction mixture. Because the terephthalic acid is insoluble in the mixture, the xe2x80x9cclearing pointxe2x80x9d of the solution can be taken as an indicator of complete or almost-complete reaction of the terephthalic acid. The endpoint is thus not well defined. Depending on reaction conditions, the acid levels of the intermediates obtained at this stage (hereinafter referred to as ester interchange, El) can vary from about 600 to less than 5 milliequivalents/kilogram (meq/kg).
The second stage when starting from terephthalic acid is also a polycondensation step, conducted at higher temperature and usually under vacuum. The variation in the acid endgroup levels after ester interchange greatly affects the polycondensation phase of the reaction and thus the resulting level of carboxylic acid end groups in the polymer. In the manufacture of poly (alkylene terephthalate) resins as currently practiced, intrinsic viscosity typically increases during the polycondensation step, with a simultaneous decrease in the acid level. This is clearly evident in the data presented in U.S. Pat. No. 5,663,281 to Brugel. The rapidity of the transition from the ester interchange step to the polycondensation step (particularly in a continuous process) also makes it difficult to control the level of carboxylic acid end groups in the product polymer.
It would be advantageous to use terephthalic acid as a reactant to form poly(alkylene terephthalate) resins, but to be able to control the level of carboxylic acid end groups as well as the intrinsic viscosity in the final polyester product. Lower acid levels in polyester are highly desirable for ensuring hydrolytic stability and melt stability. The higher acid endgroup levels are also useful, for example in blends with polycarbonate, where they serve to minimize transesterification between the polyester and polycarbonate. It would furthermore be particularly advantageous to have the ability to manufacture high molecular weight poly (alkylene terephthalate) resins having either low or high acid end group levels using the same processing equipment.
This invention relates to linear polyester resins, and in particular, to the preparation of high molecular weight, linear poly(alkylene terephthalate) resins.
A process for the preparation of high molecular weight, linear poly(alkylene terephthalate) resins comprises:
reacting an aromatic dicarboxylic acid with an excess of an alkane diol under conditions effective to reach the clearing point of the reaction;
pre-polymerizing the cleared reaction mixture under conditions effective to produce oligomers having an intrinsic viscosity measured in 60:40 phenol: 1,1,2,2-tetrachloroethane by weight at 25xc2x0 C. of less than about 0.7 deciliters/gram (dl/g) and a carboxylic acid end group number of less than about 100 milliequivalents per kilogram (meq/kg); and
polycondensing the oligomer under conditions effective to produce linear poly(alkylene terephthalate) resins having an intrinsic viscosity measured in 60:40 phenol:1,1,2,2-tetrachloroethane by weight at 25xc2x0 C. of about 1.10 to about 1.70 dl/g and a carboxylic acid end group number of about 10 to about 40 meq/kg.